Interest in fuel cells as a means of generating electricity for use in various systems has increased significantly in recent years. Generally, a fuel cell comprises two electrically conductive electrodes, an anode and cathode, which are separated from each other by an ion-conducting membrane, or electrolyte. Fuel cells use electrons that are separated from a fuel and transported through an external circuit as electricity. A number of fuel cells exist that use a variety of fuels, electrolytes, and materials for the cathode and anode, among other variables.
Hydrogen-fueled proton-exchange membrane fuel cells (PEMFCs) have been thoroughly researched and developed and provide good performance and power densities that are obtainable with low catalyst loadings. In operation of a PEMFC, hydrogen is split into protons and electrons on the anode side of the cell, with the protons permeating through a proton exchange membrane, such as Nafion®, to the cathode, while the electrons are forced through an external circuit, thus generating electricity, to the cathode. At the cathode of a PEMFC, oxygen is supplied and reacts with the hydrogen that has traveled through the polymer electrolyte membrane and the electrons that have traveled through the external circuit to form water. However, PEMFCs have little tolerance for carbon monoxide and, thus, their operation is limited when hydrogen is supplied from the steam reformation of light hydrocarbons, which is currently the primary source of hydrogen as a fuel. In addition, PEMFCs utilize expensive, precious-metal catalysts at the anode and cathode, thus making their commercialization not feasible.
Alkaline fuel cells (AFCs) are another well developed category of fuel cell technologies that have been extensively used in NASA's space shuttle program. An AFC produces electricity as a result of a redox reaction between hydrogen and oxygen. Hydrogen is oxidized at the anode producing water and electrons, while oxygen is reduced at the cathode after the electrons generated at the anode pass through an external circuit. The anode and cathode are separated by a porous matrix saturated with an aqueous alkaline solution, such as potassium hydroxide (KOH). The advantages of AFCs include the ability to operate at lower catalyst loadings and utilize a broader range of catalysts, including the less expensive metals nickel and silver. However, the use of methanol as a fuel is not suitable with the use of AFCs containing a liquid alkaline electrolyte because of the formation of carbonate that results from conversion of KOH to K2CO3 in the presence of CO2. The formation of the metal carbonate precipitate can block and destroy the electrode and catalyst layers.
Recently, considerable interest has also been shown in solid polymer membrane direct methanol fuel cells (DMFCs) due to the high energy density and reversible efficiencies of methanol (energy density, We=6.1 kWh/kg; reversible efficiency, ηrev=0.97) compared with liquid hydrogen (We=2.6 kWh/kg; ηrev=0.83). In addition, DMFCs operate at moderate temperatures and the replacement of a methanol fuel cartridge is relatively simple. There are two methods through which a DMFC may operate—proton-transport and anion-transport. In a DMFC with proton-transport, i.e., utilizing a proton exchange membrane, the reactions through an acid membrane are as follows:Anode: CH3OH+H2O→6H++6e−+CO2 Cathode: 3/2O2+6e−+6H+→3H2ONet: CH3OH+3/2O2→2H2O+CO2 
However, the use of proton exchange membranes in DMFCs present significant disadvantages, namely: (1) parasitic crossover of methanol from the anode to the cathode which leads to a lowering of cell voltage and efficiency; (2) electroosmosis of water from the anode to the cathode, which causes flooding at the cathode; and (3) reduced catalytic kinetics in the acidic environment requiring high loadings of expensive precious-metal catalysts, e.g., platinum.
As a result, various approaches to increase DMFC performance are currently being investigated, including the use of solid alkaline anion-exchange membranes (AAEMs), as opposed to the aqueous alkaline solution found in AFCs. The use of AAEMs with a direct methanol fuel source is advantageous because it combines the positive aspects of PEMs (all solid construction), AFCs (favorable electrokinetics) and DMFCs (high energy density of methanol), while also being able to minimize the disadvantages of each, including operating in the presence of carbonate species.
A fuel cell utilizing an AAEM for the direct use of methanol as a fuel has the same net reaction as using a proton exchange membrane, but undergoes the following reactions at the anode and cathode:Anode: CH3OH+6OH−→CO2+5H2O+6e−Cathode: 3/2O2+3H2O+6e−→6OH−Net: CH3OH+3/2O2→CO2+2H2O
Multiple efforts have been made to obtain functional AAEMs that optimize the potential advantages discussed above. Information relevant to attempts to address these problems can be found in U.S. Pat. No. 6,183,914, issued to Yao, et al., and, U.S. patent application Ser. No. 11/630,994 (Publn. No. 2008/0124604), filed Jun. 1, 2005, which are incorporated by reference herein. Additional efforts are described in Danks, T. N., et al., J. Mater. Chem. 2002, 12, 3371-3373; Danks, T. N., et al., J. Mater. Chem., 2003, 13, 712; J. R. Varcoe, et al., Chem. Mater., 2007, 19, 2686-2693; J. R. Varcoe, et al., Chem. Mater., 2007, 19, 2686-2693; J-S. Park, et al., J. Power Sources 2008, 178, 620-626, all of which are also incorporated by reference herein. However, each one of these references suffers from one or more of the following disadvantages: low radiation resistance, mechanical instability or degradation at elevated temperatures, and/or instability or degradation at elevated alkaline concentrations.
For example, poly(vinylidene fluoride) (PVDF) and poly (tetrafluoroethern-co-hexafluoropropylene) (FEP) have been grafted with 4-vinylbenzyl chloride, followed by modification of the benzyl chloride functionality with trimethylamine to give the trimethylbenzylammonium salt. Danks, T. N., et al., J. Mater. Chem. 2002, 12, 3371-3373. The PVDF-based AAEM degraded on subsequent amination and conversion to the alkaline form for hydroxide ion exchange. Danks, T. N., et al., J. Mater. Chem., 2003, 13, 712. The FEP-based AAEM gave conductivities of 0.02 S/cm at ambient temperatures and humidity of 100%, but was still brittle and contained tears. J. R. Varcoe, et al., Chem. Mater., 2007, 19, 2686-2693.
As noted above, these efforts have not yielded an anion exchange membrane that is stable and able to operate with reduced resistance and high conductivity at elevated operating temperatures or in highly alkaline environments typically found in a direct methanol fuel cell. In addition, the prior membranes incorporate quaternary ammonium salts as the ionophore, which experience intense cation-anion interaction with the hydroxide ions crossing the membrane, thus impeding hydroxide mobility and lowering the conductive capacity of the membrane. Finally, the prior efforts often involve complex steps for synthesis of the membrane, such as radiation grafting.